The Pentagon described the 31-year-old Bangladeshi as a senior Isil leader who had been killed as part of a drone campaign to defeat the militants by “striking at the head of the snake”.

His death near the Isil stronghold of Raqqa in Syria on December 10, came less than 18 months after he had left his home near Cardiff, where he had studied and gone on to be a well-respected director of a computer firm.

Former business associates described their shock at his death and said he had never shown signs of extremism while living in the UK.

Siful Haque Sujan's former workplace in Cardiff

One business associate, who had known him for more than 10 years, said he was shocked.

"He was an entrepreneur and he became a friend," he said.

"He helped everyone create websites for their businesses. He was always coming up with ideas. He was very intelligent and persistent but was just a normal run of the mill chap.

"He loved helping people and was friends with everybody and it didn't matter about their religion or culture."

Sujan entered the UK in 2003 and went on to study computer system engineering at the University of Glamorgan. He set up a Newport-based computer firm with his brother that provided online ordering systems.

He also held at least two patents for devising computers systems and was regarded as a pillar of the local Bangladeshi business community.

An immigration tribunal in 2014, where he unsuccessfully applied for a special visa to remain in the country because of his sought-after skills, heard he was “an individual of considerable ability”.

One former colleague described him as “thoughtful, punctual, dutiful and hard-working”, while another considered him “gentle and down to earth”.

A neighbour at the home in Pontypridd where he lived for six years said he normally wore a suit and drove high performance cars, including a black BMW.

Donna Davies, 33, said: "He seemed nice. The only time I would see him was late at night. His wife and son rarely left the house, you never saw them.”

Sujan became increasingly religious after the death of his sister-in-law and he adopted his nephew to bring him to live in Britain.

He then left Britain in July 2014 after telling friends he and his wife wanted to return to Bangladesh where his family was in the jute trade. One former friend said he was then rumoured to have travelled to Turkey to set up another business. Since his killing, at least one business associate has been questioned on Sujan’s activities by counter terrorism police.

When he arrived in Syria, Siful Haque Sujan’s computer skills were put to use in the unit developed by the British jihadi Junaid Hussain, who became Isil’s leading hacker.

Hussain also had an "outreach" role, encouraging lone wolf attacks around the world. He was in contact with two men who attacked a "Draw a Cartoon of the Prophet" event in Texas in May.

Sujan became a follower of Hussein, and when his mentor was killed in a drone strike on Raqqa on August 24, Sujan, who called himself Abu Khalid al-Bengali, seems to have taken over a number of his duties.

He was described by the Pentagon as having been involved in counter-surveillance and also in “external operations” planning – possibly a reference to his having taken over Hussain's "outreach" role too.

Sujan attended the University of Glamorgan at the same time as Aminu Sadiq Ogwuche, a suspected Boko Haram ringleader, and Norman Idris Faridi, who was jailed for downloading extremist material. It is not known if he knew either of them.

A spokesman for the university, which has since become University of South Wales, said: “In common with the UK's other major international universities, we take seriously the shared effort to counter radicalisation, and in this we work closely and appropriately with our local faith communities and the relevant agencies."